Casting something that large and intricate would be incredibly time consuming and expensive. I cast a small, roughly foot-long simple log with no branches for an advanced sculpture class at school last year and it was quite a complex process.
First, I had to make a two part mold out of a flexible casting material, I believe I used a silicon mold material that runs approximately $100 a gallon. This involved coating the entire object in a specific lubricant designed for the silicon material so that the object would eventually release once the mold was made. Next, once the object is lubricated you begin to coat the entire object with a 1/16" layer of the mold material. This layer takes approximately an hour to dry. Once it is dry, you come back and coat it with another layer then let that dry. Repeating this process, you eventually create a 1/2" mold. My log took maybe an eighth of a gallon.
Before you remove the mold from the object, you have to make a mother mold out of plaster strips to support the thin silicon mold when actually pouring your replica. Without this, the silicon mold would distort or possibly break when you actually pour your material of choice into the mold. The mother mold on my log was two part, but there is really no telling how many individual sections you would need to cast a full stump, because unlike the thin silicon material which you can flip inside out and bend to get the object out of the thin mold, the plaster mother mold must slide off directly. Each larger root might take two separate sections to cast successfully. Once the mother mold has hardened, you take these sections off the object, then attempt to remove the thin silicon mold in one piece. You might need to cut a zig-zag section called a zipper into the side to allow it to come off.
Now, finally, you actually cast the object. First, you flip the mother mold upside down and craft a wooden support structure to hold the mold in place so you can pour into the open underside of the mold. Next, you slide the silicon mold into place inside the mother mold and lubricate the inside of the silicon mold. Mix together the material you have decided to cast your object out of according the directions supplied with the material, then pour it into the mold slowly so that you fill all crevices of the mold and avoid any air bubbles. Once dried, removing the mold is the same as removing it from your original object.
This is the most efficient way that I know to cast an object like a stump, but there certainly could be easier ways, as I have relatively limited casting experience.
Considering the amount of effort and funds needed to cast a stump, I would personally suggest that it would be much easier to either carve a replica out of foam or get a couple of friends to help you carry the stump to your tank.